This invention relates to sensors, and more particularly, to capacitive proximity sensors.
Capacitive sensors using a single plate capacitive proximity detector are known. The publication entitled: Capacitive Sensors-Design and Applications (ISBN#0-7803-5351-X) by Larry K. Baxter, 1st ed. (IEEE, Piscataway, N.J., 1997) describes one known capacitive proximity sensing system. Typically, a balance is achieved such that the system is stable. The balance is disrupted, for example, when a third object projects itself into the system, thereby altering a previous capacitance. The net result is a disruption of the balance. The balance can be achieved by using a bridge circuit with the proximity detecting capacitor in one arm of the bridge and a second capacitor, that has been adjusted to null the output of the bridge, in the other arm. Alternatively, the system can consist of two virtually identical oscillators that are independent of each other. Each of the two identical oscillators generates a signal with a frequency, dependent on a capacitance, that is virtually identical to the other oscillator. Thus, when one capacitance is changed, the balance between the two frequencies is disrupted and the disruption can be measured, for example, by way of an electronic device.
To understand the instant invention, some related background is necessary. Capacitance is the linear coefficient that relates the charge induced on one electrode to the potential applied to a different electrode. There is also self-capacitance that relates to the charge on an electrode to its own potential. For example, consider a situation with N electrodes, all electrically isolated from one another. Electrode i has charge Qi and potential Vi. The charge on electrode i can be expressed as:                               Q          i                =                              ∑                          j              =              1                        N                    ⁢                      xe2x80x83                    ⁢                                    C              ji                        ⁢                                          V                j                            .                                                          (        1        )            
Here, Cji is the capacitance of electrode i with respect to electrode j. Given the geometry, the capacitance can be calculated. In electrostatics it is known that for any geometry, Cji=Cij.
An exemplary embodiment of the invention is a sensing circuit for use with a proximity sensor having a first electrode and a second electrode and a capacitance between the first electrode and the second electrode. The sensing circuit includes a signal generator for generating an alternating signal. The signal generator is for connection to the first electrode of the sensor. A synchronous detector has a first input for connection to the second electrode of the sensor. A phase shifter is coupled to the signal generator and generates a phase shifted signal in response to the alternating signal. The phase shifted signal is provided to a second input of the synchronous detector. The synchronous detector detects a change in the capacitance between the first electrode and the second electrode in response to a signal from the second electrode and the phase shifted signal. Another embodiment of the invention is method for detecting a response of a capacitive proximity sensor.